1. Field of the Invention
The present invention relates to a polishing end point detection method, a polishing end point detection apparatus, and a polishing apparatus, and more particularly to a polishing end point detection method and a polishing end point detection apparatus for detecting an end point of polishing of a multilayer structure.
2. Description of the Related Art
An integrated circuit, which is a semiconductor device, is fabricated through several processes. STI (Shallow Trench Isolation) is one of such fabrication processes, and is important in realizing a finer integrated circuit. STI is a technique of electrically isolating elements, such as transistors, formed on a wafer (typically a silicon wafer) using an insulating film.
FIG. 1 is a cross-sectional view illustrating a process of STI. More specifically, FIG. 1 shows a structure in which a SiO2 film 2 as an insulating film is embedded in grooves (or trenches) formed on a silicon wafer 1. As shown in FIG. 1, a SiN film (Si3N4) 3 and a thermal oxide pad 4 are formed between a surface of the silicon wafer 1 and the SiO2 film 2. The SiO2 film 2 is polished by CMP (Chemical Mechanical Polishing) until the SiN film 3 is exposed. This SiN film 3 is formed as an underlying layer of the SiO2 film 2. The SiN film 3 functions as a polishing stopper that prevents damage to the surface of the silicon wafer 1 which can be caused by CMP. After CMP, an etching process or the like is performed so as to remove the SiN film 3 and the thermal oxide pad 4 to thereby expose the surface of the silicon wafer 1.
Spectroscopic polishing end point detection has been known as a method of detecting a polishing end point of the SiO2 film 2. This spectroscopic polishing end point detection method monitors an interference color of a surface that is being polished, and detects a polishing end point from a change in the interference color. However, the SiO2 film 2 and the underlying SiN film 3 have a similar index of refraction, and as a result, the interference colors thereof are similar to each other. Therefore, it is difficult to detect the polishing end point from the change in the interference color using the spectroscopic polishing end point detection method.
A method using a laser light is also known as a method of detecting the polishing end point of the SiO2 film 2. This method directs the laser light to a surface of a film, and detects the polishing end point from a change in intensity of the laser light reflected from the film. However, this method has the following problems. Since the laser light is a monochromatic light, the reflected laser light disappears periodically as a thickness of the film changes. This is because a condition of interference between the laser light reflected from the surface of the film and the laser light that has passed through the film and has been reflected from a surface of an underlayer changes depending on the thickness of the film. Such a periodical change in intensity of the laser light prevents accurate detection of the polishing end point. Moreover, since circuit patterns are generally formed on the surface of the wafer, the highly-directional laser light may be reflected at an unexpected angle due to an effect of the circuit patterns. As a result, the reflected laser light cannot be received.
Ellipsometry (polarization analysis) is also known as a method of detecting a polishing end point of a film. This ellipsometry is a technique of determining an index of refraction and a thickness of a film. Ellipsometry applies a linearly-polarized light to a film on a substrate, and measures a phase difference Δ between p-polarized light and s-polarized light in the reflected light from the film and an amplitude ratio ψ of the p-polarized light to the s-polarized light. The index of refraction and the thickness of the film are determined from the measurements Δ and ψ.
The index of refraction and the thickness of the film cannot be calculated directly from the measurements Δ and ψ. Thus, in ellipsometry, an optical model is prepared in advance, and the index of refraction and the thickness are analyzed using a curve fitting or other technique. However, analyzing of the index of refraction and the thickness of the film is complex. Therefore, a relatively long period of time is required for determining the film thickness. The polishing end point detecting function of the polishing apparatus exerts a great influence on quality of a polished workpiece. In particular, it is strictly required to prevent excess polishing. In order to prevent excess polishing, it is necessary to measure a state of the film and to quickly determine the polishing end point (e.g., a point where the underlying film is exposed) so as to stop polishing.